Microstrip microwave communication antennas are known in the art. Such antennas consist of a microstrip signal radiator, often referred to as a "patch", which may take several suitable geometric configurations including a square, a rectangle, a ring or a circular disc. For most uses of such antennas, such as for mounting on transportable equipment or on vehicles, it is preferable that the antenna be thin and protrude either not at all or only very slightly from the surface on which it is mounted. Accordingly, patch antennas have heretofore been constructed with either a single layer dielectric substrate or, except for unusual applications, a pair of dielectric substrates. The prior emphasis on thinness has been at the cost of operational bandwidth and the need for empirical tuning adjustments.
Parallelogram, preferably square, shaped radiating elements are commonly used for patch antennas. In this form, the antenna constitutes essentially a pair of resonant dipoles formed, for example, by the opposite edges of the patch. Most commonly, the microstrip patch is of such dimensions that either pair of adjacent sides can serve as halfwave radiators, although the dimensions of the patch may vary so that the resonant dipole edges may be from a quarter wavelength to a full wavelength long.
Patch antennas of this type have been found particularly suitable for use in aircraft. U.S. Pat. No. 3,921,177 to Munson, for example, discloses a variety of microstrip antenna configurations adapted for such use. Patch antennas may also be used for portable hand-carried navigation equipment or on vehicles. In such cases, the microstrip antenna is part of a navigational system in which it may be necessary, for example, for the antenna to receive signals from a multiplicity of satellites located virtually anywhere overhead from horizon to horizon. For these purposes, it has been found that circular polarization of the r.f. signals is necessary and desirable, although persons of ordinary skill will recognize that circular polarization is a special case of elliptical polarization and that perfect circularity need not be achieved for effective circularly polarized propagation.
Heretofore, circular polarization of patch antennas has been achieved in a variety of ways. For example, circular polarization may be obtained when the input coupling point to the signal radiator patch is located within the interior of the patch, along a diagonal line from one corner of the patch to the other. As is well understood, this prior feed arrangement permits the exciting of a pair of orthogonal radiation modes with slightly different frequencies out of phase by 90 degrees. The required adjustment of the effective dimensions of the radiator patch to achieve exactly the 90 degree phase shift, either by slicing a thin strip off of one side of the patch or by manipulating small tabs formed on the edges of the patch as tiny tuning stubs, has been found heretofore to be both critical for proper performance and unduly costly. In addition, small variations in the dielectric constant of the substrate can have a significant effect on the resonant frequency and therefore on the degree of circular polarization achieved. Material and manufacturing processes have been known to introduce variations of as much as a few percent in the dielectric constant and fabricated dimensions of the patch from one production batch of printed antenna boards to another. These variations have the effect of detuning the antenna with respect to the desired operating frequency and require precise empirical and therefore costly post-manufacturing tuning adjustments on a unit-by-unit basis.
Various attempts have been made heretofore to overcome one or more of the foregoing disadvantages. For example, in the foregoing patent to Munson there is disclosed a square patch antenna being fed on two adjacent sides by a co-planar feed circuit which consists of a 90 degree phase shifting microstrip. Such an approach may be less sensitive to small variations in the dielectric constant of the fabricated patch board. However, antennas of the type disclosed by Munson require an exceptionally low-loss feedline and Munson describes his feedlines as generally constructed by printed circuit board techniques in which the branch line r.f. feed, impedance matching conductors and the r.f. radiator patch are arranged in a generally co-planar microstrip format. It has been found that antenna patches fed by such a feed circuit will be unacceptably lossy, in part because of radiation occurring from the microstrip feedline itself.
Such shortcomings in microstrip antennas having co-planar radiating elements and feeds have been recognized heretofore as, for example, in U.S. Pat. No. 4,054,874 which discloses reactive coupling of antenna elements. The bandwidth of the antenna structures so coupled has, however, been found heretofore to be unacceptably narrow. In addition, U.S. Pat. No. 4,554,549 to Fassett et al. discloses capacitively coupled patch antenna elements. For this purpose, Fassett et al disclose the use of up to three dielectric sheets to form a composite antenna structure of purported broad bandwidth capabilities. One of the dielectric sheets separates the feedline from the radiating antenna element. In another embodiment, Fassett et al utilize a parasitic antenna patch and associated thin dielectric sheet to overlie the antenna to provide a double-tuned response characteristic. However, Fassett et al fail to disclose a microstrip feedline associated with the ground plane in such a way as to act as a stripline without radiating. Thus, the Fassett et al. device would experience undesirable loss from the feedline circuit.
In U.S. Pat. No. 4,163,236 to Kaloi there is disclosed a corner fed microstrip antenna. Kaloi explains how to achieve circular polarization from a single feed line but does not show capacitive coupling to the radiator patch.
Accordingly, it is a principal object of the present invention to provide a high performance circularly polarized patch antenna excited by a non-radiating feed circuit which minimizes impedance mismatch and losses.
Another object of the present invention is to provide a high performance circularly polarized patch antenna which utilizes a stripline feed circuit to eliminate radiation losses.
Yet another object of the present invention is to provide a high performance circularly polarized patch antenna in which capacitive coupling is utilized to excite a square or rectangular microstrip radiator.
A further object of the present invention is to provide a high performance circularly polarized multi-layer patch antenna which is fed by an overlapping feed circuit in which coupling fingers are capacitively coupled to the radiator patch.
A still further object of the invention is to provide a high performance circularly polarized multi-layer patch antenna in which a large ground plane of at least approximately twice the size or about four times the area of the radiating patch is utilized substantially to enhance the bandwidth performance of the antenna.
A yet further object of the present invention is to provide a microstrip patch antenna capable of maintaining better than -25 dB return loss over a 40 MHz bandwidth range.